1. Field of the Invention
The present invention relates to a method for obtaining an acid solution containing nickel from a positive electrode material constituting a nickel-metal hydride battery and using a foamed nickel plate (Celmet).
2. Description of the Related Art
In recent years, environmental problems, such as acid rain attributable to acid gas of nitrogen oxide, sulfur oxide, or the like, which is emitted into the air, and global warming due to carbon dioxide gas, have been highlighted as global problems. In order to reduce contamination by exhaust gas of automobiles, one of the causes, hybrid vehicles equipped with a secondary battery such as a nickel-metal hydride battery have been attracting attention.
This nickel-metal hydride battery comprises a positive electrode, a negative electrode, an electrode terminal, and an electrolyte, as functional members, and furthermore comprises an electrode substrate, a separator disposed between positive and negative electrodes, and a case for housing these members, as structural members.
Here, each of the members comprises various materials and components such that a positive electrode active material is made of nickel hydroxide containing additional trace elements; a negative electrode active material is made of a hydrogen storage alloy containing nickel, cobalt, rare earth elements (misch metal), or the like; the electrode substrate is made of a nickel plate, a nickel-plated iron plate, or the like; the separator is made of plastics; the electrolyte is made of a potassium hydroxide solution; the electrode terminal material is made of metal such as copper, iron, or the like; and the case is made of plastics, steel, or the like.
As a structure of the nickel-metal hydride battery, an electrode body is configured such that positive electrodes and negative electrodes are alternately layered, with disposing plastics between each of the positive and negative electrodes as separators. This electrode body is placed into a case made of plastics or steel, and the electrodes and the case are connected with the electrode terminal materials made of metal, such as copper or iron, and finally an electrolyte containing a potassium hydroxide solution as a main component is filled between the electrodes, and the case is sealed.
There is a positive electrode substrate of the nickel-metal hydride battery, called Celmet, which has a structure configured that a collector is formed by using foamed nickel as a base material, and a positive active material is press-fitted therein and pressure molding is performed. Celmet has a feature that enables electrodes to be produced relatively easily and to have a high capacity more easily, and has been used widely.
A nickel-metal hydride battery installed in hybrid vehicles are replaced with new one when deteriorating with use, or removed when vehicles are scrapped, thereby becoming a spent nickel-metal hydride battery. In addition, a waste material which is not commercialized, such as a defective product and a trial product, generates in a manufacturing process of nickel-metal hydride batteries.
The spent nickel-metal hydride battery and the waste material (hereinafter, collectively referred to as a nickel-metal hydride battery or the like) contains many kinds of rare and valuable metals, such as nickel, cobalt, and rare earth elements, and therefore recovering and reusing these valuable metals have been attempted.
However, nickel-metal hydride batteries have a complex and solid structure, and furthermore, comprise materials many of which are chemically stable. Therefore, it was not easy that metals contained in a nickel-metal hydride battery or the like, such as nickel, cobalt, and rare earth elements, were separated and recovered to be reused as materials for new batteries.
Therefore, as a measure to solve the problem, for example, there has been proposed a method for recovering a metal from a nickel-metal hydride battery or the like, the method wherein a nickel-metal hydride battery or the like is placed into a furnace to be melted, and plastics constituting the battery are combusted to be removed, and furthermore, most of iron is made into slag to be removed, then nickel is reduced and recovered as ferronickel in which a part of iron is alloyed.
This method has a feature of little investment and less time and effort for treatment, such as a easiness to utilize existing smelting works and existing refining equipment. However, the recovered ferronickel also contains a large amount of impurity elements and is not suitable for any use other than a stainless raw material. In addition, most of cobalt and rare earth elements are distributed into slag and discarded to the outside of a system, and accordingly are not usable effectively, hence this method is not desirable.
As referred in Japanese Patent Publication No. 3918041, there has been proposed a method for recovering a metal from a spent nickel-metal hydride storage battery, the method comprising the steps of: forming an aqueous phase by dissolving a storage battery scrap with acid; separating rare earth metals from the aqueous phase as a double sulfate; then precipitating iron from the aqueous phase by raising pH; performing liquid-liquid extraction of a filtrate obtained after the iron precipitation by using an organic extractant to separate zinc, cadmium, manganese, aluminum, and residual iron and rare earth elements, wherein the extractant and the pH value is selected so that, after the extraction, substantially only nickel and cobalt are dissolved into the aqueous phase and remain at the same atomic ratio as at the time of having existed inside the storage battery scrap; then, precipitating a nickel or cobalt alloy from the aqueous phase; and finally using the nickel/cobalt alloy as a master alloy in order to produce a hydrogen storage alloy.
However, in this method, it is not easy that nickel and cobalt are electrodeposited as an alloy so as to have exactly the same ratio as in battery composition, and there is a possibility that, depending on solution composition and electrolytic condition, electrodeposited alloy composition might change. Therefore, in order to accomplish an exact alloy composition, it takes extra time and effort to analyze an obtained alloy each time, and then to add and redissolve an insufficient component in a required amount.
Furthermore, it is known that battery characteristics change depending on alloy composition, and the alloy composition is kept changed and improved by such as adding a new component in order to improve performance of a battery, and therefore, a recovered nickel alloy and a recovered cobalt alloy were not necessarily reusable as they were.
In addition, when a nickel-metal hydride battery or the like is leached as it is by using an acid, or an acid and an oxidizing agent, a large amount of the acid or the oxidizing agent is consumed only for neutralizing potassium hydroxide, which is an electrolyte component. Note that, when sulfuric acid is used for leaching, a double sulfate of potassium originating in the electrolyte and a rare earth element contained in an electrode active material generates and precipitates, whereby the rare earth element could become a loss.
Furthermore, in this method, by setting a magnetic fraction step, performed is a treatment wherein trivalent iron ions contained in a leachate are recovered to divalent iron ions by using metallic iron. However, since a large amount of sulfuric acid exists in the leachate, there is a possibility that more than an equivalent of iron to recover trivalent iron ions dissolves excessively, whereby an iron concentration in the leachate increases excessively.
Therefore, there has been a problem that an amount of a neutralizing agent used and an amount of precipitate generated in a deferrization step increase, whereby cost is raised, and nickel which simultaneously coprecipitates with iron to become a loss increases.
Hence, there has been sought a method for efficiently obtaining a solution containing nickel and cobalt from a positive electrode material of a spent nickel-metal hydride battery or the like.
The present invention provides a production method for obtaining a solution with a high content of nickel and cobalt by more simply and more efficiently separating nickel and cobalt from a positive electrode material, a production waste material, or the like, of a spent nickel-metal hydride battery.